fix(reddit): switch to Exa-only for search and read, remove proxy dependency

Reddit blocks nearly all non-browser access at IP level (including ISP proxies).
Switch Reddit channel to use Exa exclusively:
- Search: web_search_exa with includeDomains: ["reddit.com"]
- Read: crawling_exa for full post + comments
- check() now verifies Exa availability instead of probing Reddit API
- tier changed from 1 (needs config) to 0 (zero config)
- Removed reddit_proxy from config, CLI, and setup guide
- Updated all docs (README, README_en, install.md, SKILL.md, references)
- Fixed xreach→bird references in references/social.md

Fixes #218, Fixes #222, Fixes #221

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent_reach/skill/SKILL.md

@@ -63,6 +63,12 @@ bird search "query" -n 10
 # YouTube/B站字幕
 yt-dlp --write-sub --skip-download -o "/tmp/%(id)s" "URL"
 
+# Reddit 搜索（通过 Exa，免费无需代理）
+mcporter call 'exa.web_search_exa(query: "query", numResults: 5, includeDomains: ["reddit.com"])'
+
+# Reddit 读帖（通过 Exa）
+mcporter call 'exa.crawling_exa(urls: ["https://www.reddit.com/r/.../comments/.../"], maxCharacters: 10000)'
+
 # V2EX 热门
 curl -s "https://www.v2ex.com/api/topics/hot.json" -H "User-Agent: agent-reach/1.0"
 ```